Necrotic ring spot of turf

Crystal Floyd

Necrotic ring spot, formerly part of the disease complex Fusarium blight, is caused by the fungus Leptosphaeria korrae. Necrotic ring spot frequently occurs on two- to five-year-old sodded lawns and is especially prevalent in vigorously growing lawns. These lawns often develop a thick thatch layer, which stimulates fungal growth. Necrotic ring spot is also common in lawns that have layered soil; one to two inches of topsoil laid down over hard compacted native soil, but not mixed together. This effect produces a lawn with shallow roots and poor drainage.

Kentucky bluegrass and fescue are the most common species of grass infected. Symptoms of necrotic ring spot are similar to those of other lawn diseases, especially yellow patch. Thus, necrotic ring spot is a difficult disease to diagnose. To differentiate between these two diseases, a laboratory examination is necessary.

The fungus survives winter in infected plant material. The disease is spread by plant to plant contact or mechanically with lawn equipment. The fungus becomes active during cool, wet periods in the spring and again in the fall. Grass sheaths and roots are infected and eventually killed. The first symptoms, which are rarely observed, are reddish brown to straw colored grass blades. Usually, the disease is not detected until many plants are killed, forming small circular patches of dead, straw-colored grass. The center of the patch is often recolonized by healthy grass, creating rings of dead grass that resemble "frog-eyes" or "donuts." Multiple rings often coalesce, forming irregular patterns in the lawn. Damage is most evident in the summer, even though the fungus is not active at this time. During hot, dry weather, damaged root systems can no longer support the plant and the entire plant dies.

Necrotic ring spot can be problematic, but implementation of proper cultural practices can manage this disease. Lawns that are compacted or have thick thatch layers promote the development of necrotic ring spot. Fungicides are available that manage the disease preventively, but are of limited use once disease symptoms are visible. However, if cultural deficiencies are not corrected, necrotic ring spot may persist in a lawn for many years.

Dethatching: Dethatch in the spring or fall (preferably fall) if the thatch layer is over ½" thick. Dethatching may need to be repeated for several years to reduce the thatch layer.

Aerating: Aerate in the fall with a core aerator that removes plugs of dirt from the ground. Aeration will reduce compaction, increase nutrient and water penetration, and increase microbial activity, which helps decompose the thatch layer.

Fertilizing: Properly fertilize lawns by following the recommendations of a soil test. Tests are available from the U of MN Soil Testing Lab (612-625-3101). Increased nitrogen fertility (2-3 times per year) in the form of a slow release fertilizer, such as Ringer Turf Restore, will help reduce disease.

Watering: Water throughout the summer, frequently and lightly due to shallow root systems. Water three to four times per week for a total of one inch (including rainfall). In addition, a light mid-day syringe cycle (watering just enough to wet the leaves) can help the plants tolerate heat stress during days with excessive temperatures (+85° F). After the grass recovers, or in the fall when conditions are cool and wet, return to a regular watering schedule of one inch per week or ½" two times per week, depending on soil type and amount of rainfall.

Mowing: Mow grass at a height of 2½" to 3½" in the summer. Mow frequently and never remove more than one inch of growth at a time.

Reseeding: Dead rings of grass will likely recover in the fall, when plants are not experiencing heat and drought stress, and when fungal activity subsides. However, poor cultural conditions will stimulate the disease causing rings to develop again in the spring. Reseeding dead areas with resistant varieties of bluegrass may expedite the recovery process. Many varieties of resistant Kentucky bluegrass are available including Adelphi, America, Eclipse, Kelly, and Midnight. Be cautious when using weed control products during reseeding, as most can damage grass seed. Resodding damaged areas has not been successful. New sod is often reinfected.

Fungicides: Fungicides will not cure damaged grass. Fungicides are only applied as preventative treatments when the soil temperature, at a two-inch depth, reaches 65° F. This is usually late May to early June. Make a second application one-month later. A preventative treatment may also be applied in the fall (usually September), but spring applications are much more effective. To achieve greatest activity, fungicides should be watered in after application to move the product to the root zone. Fungicides registered for necrotic ring spot contain thiophanate-methyl (Bonomyl or Cleary's 3336). Fungicides will most likely need to be applied for several years until disease symptoms subside.

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